1. Field of the Invention
This invention relates to a process for producing .alpha.-olefin polymers and more particularly it relates to a process for producing .alpha.-olefin polymers with a controlled stereoregularity and with a higher yield.
2. Description of the Prior Art
It is well known that .alpha.-olefins are polymerized by means of the so-called Ziegler-Natta catalysts consisting of a compound of metals of Groups IV-VI of the Periodic Table and an organometallic compound of metals of Groups I.about.III of the Table, including those modified by adding an electron donor, etc. It is also known that propylene or the like is subjected to stereoregular polymerization using particularly titanium trichloride-containing compositions among the Ziegler-Natta catalysts (e.g. Japanese patent publication No.Sho 32-10596, etc.). In this case, the stereoregularity of the resulting .alpha.-olefin polymers has heretofore depended on titanium trichloride-containing compositions or catalyst components having modifiers such as electron donors incorporated therein and polymerization conditions, and it has been impossible to optionally control the stereoregularity of homopolymers.
If the stereoregularity of polypropylene is expressed by the ratio of absorbances at 995 cm.sup.-1 and 974 cm.sup.-1 (this ratio will hereinafter be often expressed by "IR-.tau."; see J. P. Luongo, Journal of Applied Polymer Science, 3, 302 (1960)), values of IR-.tau. of polymers obtained using titanium trichloride-containing compositions have been usually in the range of 0.93 to 0.95. Processes for controlling such IR-.tau. values have been carried out by varying the polymerization temperature or adding an additive or copolymerizing propylene with another .alpha.-olefin as a comonomer. For example, processes for reducing the IR-.tau. values down to about 0.83 to 0.93 in order to reduce the toughness of polymers and elevate the impact strength thereof, have been carried out by elevating the polymerization temperature or adding an additive such as trialkylaluminums or copolymerizing propylene with a comonomer such as ethylene, butene-1. However, such prior art has had a drawback of byproducing a large amount of atactic polymer, resulting in great reduction in the toughness of the resulting polymer, decrease in the tensile strength thereof or a product having a sticky surface to thereby degrade the physical properties of the product. Thus it has been necessary to remove increased atactic polypropylene. As a result, it has been necessary to further install equipments for removing atactic polymer and treating the thus removed polymer e.g. by combustion treatment, resulting in an inferior polypropylene yield and an increase in the polymer production cost.
Further, even when the stereoregularity of polybutene-1 has been given in terms of the X-ray crystallinity according to conventional method, it has so far been impossible to optionally control the stereoregularity.
If it is possible to utilize such a catalyst that when its catalyst components can be varied by a simple operation in the catalyst preparation, such a variation varies the stereoregularity of the resulting polymer, then it becomes possible to produce the polymer by controlling the stereoregularity; and by employing such a catalyst, it becomes easy to produce various polymers which are different in the physical properties such as toughness, stretching characteristics, etc., by the use of the same plant, and this is advantageous in the effective use of plants and also in broadening the application fields of polymers.
The present inventors have made studies on titanium trichloride-containing compositions defined in the present invention, and as a result have found that in the preactivation (i.e. preliminary activation) of the compositions, a reaction product of a trialkylaluminum with an electron donor is added and also the molar ratio of these two reaction raw materials are varied in the formation reaction thereof, whereby it is possible to control the stereoregularity (IR-.tau.) of polymers finally obtained, and have attained the present invention.
The object of the present invention is to provide a process for producing .alpha.-olefin polymers which is easy to control the stereoregularity of polymers finally obtained, without any increase in the amount of atactic polymers.